Evidence for a large fraction of Compton-thick quasars at high redshift

Abstract

Using mid-infrared and radio selection criteria, we pre-select a sample of candidate high-redshift type 2 quasars in the Subaru XMM-Newton Deep Field (SXDF). To filter out starburst contaminants, we use a Bayesian method to fit the spectral energy distributions (SEDs) between 24-μm and the B-band, obtain photometric redshifts, and identify the best candidates for high-z type 2 quasars. This leaves us with 12 z_phot >= 1.7 type 2 quasar candidates in an area ~0.8 deg², of which only two have secure X-ray detections. The two detected sources have estimated column densities N_H ~ 2 & 3 × 10²⁷ m^-2, i.e. heavily obscured but Compton-thin quasars. Given the large bolometric luminosities and redshifts of the undetected objects, the lack of X-ray detections suggests extreme absorbing columns N_H >~ 10²⁸ m^-2 are typical. We have found evidence for a population of `Compton-thick' high-redshift type 2 quasars, at least comparable to, and probably larger than, the type 1 quasar population, although spectroscopic confirmation of their active galactic nuclei nature is important.